Simple UV Mesh Unwrapping Scale issue
If you scaled you box in Object mode you will need to apply the scale to get the faces the right size. To apply the scale Ctrl A -> Scale

You need to add seams to properly unwrap your mesh. If you do not use seam and try to unwrap you will get a weird and distorted UV map. To add a seam you can select and edge Ctrl E -> Mark Seams. 



After you add enough seems to cleanly separate faces in the correct places you can unwrap the mesh, U -> Unwrap. Make sure you select all polygons before unwrapping.